Your own textures... Hm.
Actually painting the shadows yourself would be a major pain in the ass, and REALLY hard to get right.
Here's one thing you can do:
First make/find the texture you want on the skin.
Resize it to a good size.
Copy it and paste it into the skin.
This should have made a new layer with only the texture on it.
Here you can paste more copies of the texture or add other ones and align them so that they cover the area you want.
Then change the layer opacity to about 30-50% so that you can see both the skin and the texture.
Use the lasso tool to remove the extra parts from the texture layer so that it fits properly on the skin.
Then fiddle a bit with the opacity again to get the right look.
This is a really quick camo that I made this way, it looks decent to me.
http://img78.photobucket.com/albums/v291/TemporaryTomato/Transparent.jpg)
Happy now? :p